U.S. Pat. No. 5,652,021 describes a flame-based deposition technique termed combustion chemical vapor deposition or “CCVD.” U.S. Pat. No. 5,997,956 describes a CCVD process using near-supercritical fluid solutions. U.S. patent application Ser. No. 09/067,975 describes apparatus and process for “Controlled Atmosphere Chemical Vapor Deposition” or “CACVD.” The teachings of each of the above-mentioned U.S. Patents and Applications are incorporated herein by reference. The techniques taught in these patents and applications provide for large-scale open-atmosphere deposition of a variety of materials as thin layers on various substrates and also provide for production of powders of fine, generally uniform size. While applicants find particular utility for the needles of the present invention to promote atomization of liquids for CCVD and CACVD processes, and while the invention will be described herein to a substantial degree relative to these processes, it is to be understood that atomization using constricted needles in accordance with the present invention is applicable to a variety of processes where aerosols are used and which can benefit from control of aerosol formation, such as uniformity of flow and droplet size.
Uniformity and reproducibility of coating or powder production in a CCVD or CACVD process depends upon the ability to provide aerosols of known and reproducible characteristics, such as flow rate and droplet size.
The above-referenced U.S. Pat. No. 5,997,956 teaches CCVD processes in which “near-supercritical” fluids are atomized, such fluids being atomized at temperatures near to the supercritical temperature (Tc), e.g., within about 50° C. of supercritical. Such fine atomization is also important for other applications, such as in the fields of combustion, chemical processing, spraying, particle pressure regulation, analysis, catalysis, desiccation, and powder formation. While the atomization of supercritical or near-supercritical liquids produces aerosols of fine droplet size, the high pressures and high temperatures involved in some cases make it difficult to control the atomization so as to maintain a uniform aerosol. The nozzle of CCVD atomizers is typically a needle of very narrow inner diameter, i.e., in the range of 30 to 500 microns. When operating at near supercritical temperatures, problems can occur if the fluid crosses the liquidus and gasifies within the needle. If the liquid gasifies the dissolved solids, such as precursor(s), may precipitate within the needle, resulting in non-uniform deposition and eventually clogging the needle. Also, gasification of the liquid may result in oscillation within the needle causing cyclical flow variations. The resulting pulsed atomizing can cause significant variations in the proceeds and result in non-uniform product.
The present invention is directed to reducing the tip inner diameter of narrow inner diameter needles, preferably by at least about 25%, more preferably at least about 50% and even more preferably in many instances at least about 70%. The reduced tip diameter needles help to maintain a back pressure that increases the boiling point of the liquid in the needle and thereby prevents premature gasification of the liquid, resulting in more uniform depositions.